Description

Citation

Abstract

Mutualistic interactions almost always produce
both costs and benefits for each of the interacting
species. It is the difference between gross benefits and
costs that determines the net benefit and the per-capita
effect on each of the interacting populations. For example,
the net benefit of obligate pollinators, such as yucca
and senita moths, to plants is determined by the difference
between the number of ovules fertilized from
moth pollination and the number of ovules eaten by the
pollinator’s larvae. It is clear that if pollinator populations
are large, then, because many eggs are laid, costs
to plants are large, whereas, if pollinator populations
are small, gross benefits are low due to lack of pollination.
Even though the size and dynamics of the pollinator
population are likely to be crucial, their importance
has been neglected in the investigation of mechanisms,
such as selective fruit abortion, that can limit costs and
increase net benefits. Here, we suggest that both the
population size and dynamics of pollinators are important
in determining the net benefits to plants, and that
fruit abortion can significantly affect these. We develop
a model of mutualism between populations of plants
and their pollinating seed-predators to explore the ecological
consequences of fruit abortion on pollinator
population dynamics and the net effect on plants. We
demonstrate that the benefit to a plant population is unimodal
as a function of pollinator abundance, relative to
the abundance of flowers. Both selective abortion of
fruit with eggs and random abortion of fruit, without
reference to whether they have eggs or not, can limit
pollinator population size. This can increase the net
benefits to the plant population by limiting the number of eggs laid, if the pollination rate remains high. However,
fruit abortion can possibly destabilize the pollinator
population, with negative consequences for the plant
population.